It can be obtained from Xeroth by accepting Angel leaf study. He will give you 1 Angel Leaf Extract in exchange for 10 Angel Leaves.
all of his quests seem to be repeatable, and I got it the 2nd time the angel leaf extract quest appeared, first time I didn't get anything. I guess just wait a few days and come back to him. (I also increased my "like" and "love" level with him, not entirely sure if that has anything to do with it, I doubt it though.)